I can't comment on the X thing as I'm not familiar with any of it, but the fontconfig thing *is* familiar

.
That's a simple version mismatch: the config format evolved in some release long ago, which made newer configs partly unreadable by older versions. Or vice-versa.
This was fun for a while even on desktop, with apps that bundle fontconfig and/or fontconfig shards (looking at you, CEF & Electron

).